1. Field of the Invention
The present invention relates to a breather device, a liquid tank, and an exhaust gas purifying apparatus that is adapted for purifying exhaust gas from an engine. In particular, the present invention relates to a technology for enabling it to prevent urea precipitation inside a breather device provided for a liquid tank in which urea aqueous solution is stored.
2. Description of the Related Art
As a technology that has been developed for purifying nitrogen oxides (hereunder, referred to as NOx) exhausted from a diesel engine, by using after treatment, the urea SCR (Selective Catalytic Reduction) is known. This urea SCR adds ammonia to exhaust gas, and with this ammonia as a reducing agent, reduces and purifies the NOx. In order to accelerate the NOx reduction, a reducing catalytic converter is disposed in the exhaust gas passage. In consideration of storage convenience, urea serving as an ammonia precursor, is stored in a tank in an aqueous solution state, and in actual operation, the urea aqueous solution is supplied from this tank and injected into the exhaust gas passage upstream of the reducing catalytic converter (Japanese Laid-open (Kokai) Patent Application Publication No. 2000-027627, Paragraph No. 0013). The technology using this urea SCR is applied to exhaust gas purification in a diesel engine mounted on a large vehicle such as truck, and is being put to practical use.
Here, in the tank for storing the urea aqueous solution, in addition to a supply pipe for supplying the stored urea aqueous solution to a feed pump, there is provided a breather pipe for relieving pressure inside the tank. By introducing atmospheric air through this breather pipe when supplying the urea aqueous solution, the pressure inside the tank can be kept constant.
However, in this tank having the breather pipe, there is a problem in that if the tank vibrates due to vibrations transmitted from the engine, or traveling on a corrugated road or the like, and sloshing occurs in the stored urea aqueous solution, so that the urea aqueous solution flows along the inner wall of the tank, or splashes, and consequently attaches to the breather pipe. If the attached urea aqueous solution is left attached, this urea aqueous solution dries out due to contact with air from outside, causing precipitation of the urea, and with further repeated attachment and precipitation, the precipitated urea is accumulated, to thereby partially or completely clog the breather pipe interior. It may be considered to attach a cover provided with a vent hole in the vicinity of an opening of the breather pipe, to prevent attachment of the urea aqueous solution to the breather pipe. However, in spite of provision of the cover, if the urea aqueous solution were actually attached to the breather pipe, precipitation of urea from this urea aqueous solution could not be avoided. Moreover, when, for example, stainless steel is adopted for the material of the breather pipe owing to the necessity to have sufficient endurance with respect to the urea aqueous solution, the attached urea aqueous solution to the breather pipe flows along the seam welded part thereof that is formed when joining the stainless steel material in a cylindrical shape by the conventional induction welding, so that the urea aqueous solution is concentrated at the end of the seam welded part, with the result that blockage caused by precipitation is promoted. This is not limited to a tank for storing urea aqueous solution, but also in a liquid tank in general, if liquid attached to the breather pipe is left attached, smooth introduction of atmospheric air through the breather pipe is obstructed.